(1) Field of the Invention
The present invention relates to an image forming system, such as a digital copier or a laser printer, which has an imaging data generator to generate a dot intensity data that is used to modulate an emission power and/or an emission time of a laser diode, so that the laser diode emits a laser beam to a photoconductive medium in accordance with the modulation data. In the image forming system, an electrostatic image is formed on the photoconductive medium by exposing the photoconductive medium to the laser beam, and then a multi-level image is created on the copy sheet from the electrostatic image through a known electrophotographic process.
(2) Description of the Related Art
An image forming system of an electrophotographic type, such as a digital copier or a laser printer, scans a laser beam across an electrostatically charged photoconductive drum while the drum rotates. The areas hit by the laser beam lose their charge, and the electrostatic charge remains only where the copy is to be white. A powdered toner electrostatically adheres to the discharged areas of the photoconductive drum, and is then transferred to the copy sheet to create an image thereon. This process may be called negative/positive (N/P) process, and many image forming systems utilize this N/P process to form an image on the copy sheet.
In a conventional image forming system of the above type, the diameter of the laser beam emitted by the optical writing unit of the image forming system is usually larger than the size of the ideal dot for one pixel. By using the N/P process, the photoconductive drum is scanned by the laser beam and the powdered toner electrostatically adheres to the discharged areas of the photoconductive drum. In such a case, the 1-pixel-thick vertical black line which corresponds to that of the original image may become too broad, and the 1-pixel-thick vertical line is somewhat broader than the 1-pixel-thick horizontal line due to the laser beam profile in the electrophotographic process. Further, the multipixel-thick black line which corresponds to that of the original image may have jaggies at its edges or may be converted into a few 1-pixel-thick black lines separated by a narrow white area. In such a case, the reproductivity or fidelity is degraded. The above problems becomes substantial if the copy image is further reproduced on another copy sheet.
If the imaging conditions of the conventional image forming system using the N/P process are determined so as to fit the 1-pixel-thick vertical line, the 1-pixel-thick horizontal line which corresponds to that of the original image will become too thin. Conversely, if the imaging conditions of the conventional image forming system are determined so as to fit the 1-pixel-thick horizontal line, the 1-pixel-thick vertical line which corresponds to that of the original image will become too broad. There is a difficulty in suitably determining the imaging conditions of the conventional image forming system that achieve good reproductivity of the 1-pixel-thick line.
Japanese Laid-Open Patent Application No.5-75816 discloses an image forming technique aimed at improving reproductivity of a single dot in a two-level image. The image forming technique of the above publication determines a dot intensity of a subject pixel by referring to the intensities of the subject pixel and its neighboring pixels, and it may prevent the occurrence of a too-broad black line or a too-thin white line in the two-level image. However, the image forming technique of the above publication is applied to the improvement of the reproductivity of a dot for a two-level image, and it is difficult to apply this technique to the improvement of the reproductivity of a 1-pixel-thick vertical line or a multipixel-thick-line edge for a multilevel image.
Japanese Laid-Open Patent Application No.6-89338 discloses an image forming technique which determines a phase data (or a start position of the laser beam emission per dot) of a subject pixel by referring to the relation of the intensities of the subject pixel and its neighboring pixels in a multilevel image. It is aimed at achieving good reproductivity of a character or a row of thin lines for a multilevel image. However, it is difficult to apply this technique to the improvement of the reproductivity of a 1-pixel-thick vertical line or a multipixel-thick-line edge for a multilevel image. Also, this technique does not take account of the above problem that the 1-pixel-thick vertical line is broader than the 1-pixel-thick horizontal line in the resulting multilevel image.